Intensity of Light reflected from Coloured Surfaces. 65 



Supposing the luminosity of a vermilion-coloured surface had 

 to be compared with a white surface when both were illumi- 

 nated, saj, by gas-light, the following procedure was adopted : — 

 A space of such a size was cut out of black paper that one 

 side was rather less than twice the breadth of the rod used 

 to cast a shadow. One half of the aperture was filled with 

 a white surface and the other half with the vermilion-coloured 

 surface. The light L was caused to illuminate the whole of 

 this, and the rod R placed in such a position that it cast a 

 shadow on the white surface, the edge of the shadow beino- 

 placed accurately at the junction of the vermilion and white 

 surfaces. A flat unsilvered mirror M was placed at such a dis- 

 tance and at such an angle that the light it reflected cast a 

 second shadow on the vermilion surface. Between R and L 

 were placed the rotating sectors A B. The white strip was then 

 caused to be evidently too dark and then too light by altering 



^f::,.^^ 



7m: 



the aperture of the sectors, and an oscillation of diminishing 

 extent was rapidly made till the two shadows appeared equally 

 luminous. A white screen was substituted for the vermilion, 

 and again a comparison made. The mean of the two sets of 

 readings of angular apertures gave the relative value of 

 the two luminosities. It must be stated, however, that if 

 the screen remained open, as represented, the values would 

 not be correct, since any diffused light which might be in the 

 room would relatively illuminate the white surface more than 

 the coloured one. To obviate this the receiving screen was 

 placed in a box in the front of which a narrow aperture was 

 cut just wide enough to allow the two beams to reach the 

 screen. An aperture was also cut at the front angle of the 

 box through which the observer could see the screen. When 

 this apparatus was adopted its efficiency was seen from the 

 fact that when the apertures of the rotating sectors were 

 closed the shadow on the white surface appeared quite black, 

 which it would not have done had there been any diffused light 

 in any quantity present within the box. The box, it may be 

 stated, was blackened inside and was used in a darkened 



Fhil. Mag. S. 5. Vol. 27. No. 164. Jan. 1889. F 



